Microstructural investigation of defects in drill pipe

A piece of damaged drill pipe was investigated as to know the cause of the damage in the present case. Visual examination revealed severe pitting on the outer surface whereas the inner surface did not show any kind of pitting. Also, pitting occurred in between the grip mark areas only without attacking the grip marks themselves. Therefore it is believed that the corrosive medium was stagnant in between the grips thereby the attack could take place. SEM surface studies of the damaged drill pipe revealed pitted regions containing lot of corrosion product deposits. EDS microanalysis of the corrosion product deposits indicated presence of Fe, Mn, C (from base metal), Ca, O, S, Cl (high amounts) and Na, Al (small amounts) from extraneous source and silica from soil. The microstructure, chemical composition and hardness of the material used for the damaged drill pipe confirm to the API standards thus indicating the pipe material was healthy. Based on the above findings, it is concluded that the damage (severe pitting containing corrosion products) took place as a result of chloride attack in the presence of oxygen and it was believed that chloride ions in mud (drilling fluid) entered the pits as drill pipes are generally dunked in the mud containing dissolved oxygen and chloride ions during drilling. It was recommended that the sources of the chlorides be eliminated or concentration of chloride ions be reduced or addition of corrosion inhibitors or else proper coatings should be used on the drill pipe surfaces as the present paints were found ineffective.